1. Field of the Invention
The present invention relates to a multilayer ceramic substrate and a method for producing the multilayer ceramic substrate, and more particularly, to an improvement to increase the strength of a multilayer ceramic substrate including a cavity.
2. Description of the Related Art
Known methods for producing a multilayer ceramic substrate include, for example, a method described in Japanese Patent Application Laid-Open No. 2003-273513. In Japanese Patent Application Laid-Open No. 2003-273513, in order to solve the problem that a relatively high degree of shrinkage may occur at a location further away from an open end of a cavity, in which the shrinkage suppression effect produced by an outer constraining layer is weakened, to undesirably deform the cavity when a non-shrinkable process is used to produce a multilayer ceramic substrate including the cavity, a firing step is performed with a raw stacked body sandwiched between outer constraining layers including an inorganic material powder to suppress shrinkage while forming a constraining interlayer including the inorganic material powder to suppress shrinkage along a ceramic green layer of the raw stacked body to define a multilayer ceramic substrate, which is located at a location at which the cavity formed.
According to the production method described in Japanese Patent Application Laid-Open No. 2003-273513, in the firing step, the shrinkage suppression effect produced by the constraining interlayer acts in addition to the shrinkage suppression effect produced by the outer constraining layer, thereby substantially preventing shrinkage in the direction of the principal surface of the ceramic green layer, and enabling a multilayer ceramic substrate to be obtained without undesirable deformations of the cavity.
However, the multilayer ceramic substrate including a cavity has a problem in that a bottom wall portion defining the bottom of the cavity is likely to crack or break.
As the size of electronic devices including a multilayer ceramic substrate is reduced, the thickness of the multilayer ceramic substrate is required to be reduced. Therefore, particularly in the case of a multilayer ceramic substrate including a cavity, the thickness of the bottom wall portion must be reduced to achieve the reduction in the thickness of the multilayer ceramic substrate when the size of a mounted component to be disposed in the cavity is determined. Alternatively, when a peripheral portion defining the periphery of the cavity must be increased in height in order to accommodate mounted components having various sizes and shapes in the cavity, the bottom wall portion must be made thinner due to the increase in the height of the peripheral wall portion. As a result of these circumstances, the bottom wall portion is likely to be broken, and preventing such a break is a big issue.
In addition, the multilayer ceramic substrate including a cavity does not have a uniform thickness, i.e., the multilayer ceramic substrate has a relatively thin bottom wall portion defining the bottom of the cavity and a relatively thick peripheral wall portion defining the periphery of the cavity, makes it more likely to have undesirable deformations, such as warpage caused by firing. In this case, depending on the relationship between the thickness of the bottom wall portion and the height of the peripheral wall portion, deformations, such as warpage, may be more significantly produced in some cases. Therefore, when deformations, such as warpage, are to be prevented, the degree of design freedom of the multilayer ceramic substrate may be severely limited.